ok so when ur an op u can control a teams turret and use it anywhere but i cant seem to use it unless im right behind it, so how do i do this?

Iron-sight from far away

This has nothing to do with this thread, but Mr. Yokudan I'm assuming from your sig your a problem solver... If so theres this french-canadian guy that continues to try to molest my dog. If you would be so kind.

Once you plant the box you can use that turret from anywhere. Just look directly at it (even through walls) and you should see a prompt to Press X (or whatever your 'Use' button is)

you can spot it through walls so back up find a spot to hide sweep slowly till its outlined it yellow and operate

Just to eliminate any confusion, you have to place your receiver on the Turret from directly next to it, but once it's placed you can do as others have said to actually control it.

I usually remain relatively close to the turret when controlling it (though behind some kind of cover). Toss caltrop grenades in the enemy's oncoming path before gunning them down with the turret, damages them beforehand, saves you ammo (or 'heat' on the turret, rather), and gets you more points.